This invention relates to pistons for internal combustion engines and more particularly to an improved construction for a piston having separate head and skirt portions.
In some internal combustion engines, it is desirable to use pistons having a skirt portion which is separate from, and rotatable with respect to, a head portion. A connecting rod is attached in a conventional manner to a wrist pin within the piston. The piston head is then attached to the wrist pin on either side of the connecting rod to permit rotational movement between the connecting rod and the piston head. The ends of the wrist pin engage bushing or bearing surfaces on the skirt to also permit rotational movement between the skirt, the head and the connecting rod. A piston of this type has several advantages over a piston having integral head and skirt portions. Side thrust exerted by the connecting rod on the piston is applied only to the skirt. Elimination of side thrust on the piston head reduces rocking action of the head caused by side thrust. This in turn reduces oil consumption and also reduces noise and wear. Since the piston head is separate from the skirt, heat is not transferred by conduction from the head to the skirt. Therefore, the separate skirt operates at a lower temperature than a skirt which is integral with a piston head. The lower skirt temperature permits closer tolerances between the piston skirt and an engine cylinder, which also reduces noise.
Pistons of the above-described construction having a separate head and skirt are sometimes used in high-power diesel engines. The output of a diesel engine can be greatly increased through the use of a supercharger. However, supercharged diesel engines stress conventional pistons to their design limit due to increased combustion pressure and a consequent heat buildup in the piston. Under these conditions, the wrist pin tends to deflect or bend. In extreme cases, deflection of the wrist pin will result in a cracked piston and particularly in a cracked piston skirt.